The present invention relates to processes for treating iron-containing waste streams. More particularly, this invention relates to treating waste streams that arise from the chlorination of certain raw materials that contain titanium, and producing metal compounds from these waste streams.
Titanium ores and slags, as well as other sources of titanium typically contain many elements other than titanium itself. In order to obtain useful titanium products from these sources, generally one must remove or substantially reduce the amount of these other elements. These other elements may be referred to as xe2x80x9cimpurities.xe2x80x9d By way of example, these impurities may contain one or more of the following substances: iron, manganese, chromium, vanadium, aluminum, niobium, magnesium, silicon, zirconium and calcium.
One method for treating titanium ores, slags and other sources of titanium is the chloride process. This process, which involves the chlorination of titanium-bearing raw materials to form titanium chloride, and the oxidation of that substance to form titanium dioxide, is well known to persons skilled in the art. When materials that contain a selection of the aforementioned impurities are subjected to the chloride process, chlorides of these elements, as well as some oxychlorides may be generated.
Another method for treating titanium-bearing raw materials is the sulfate process. In this process, titanium bearing raw materials are dissolved in sulfuric acid to form titanyl sulfate, which in turn is subjected to hydrolysis and calcination to form titanium dioxide. The sulfate process is also well known to persons skilled in the art.
Historically, in the chloride process, after chlorination, the chlorinated impurities have been separated from the titanium chloride, and they have been treated as waste. For example, some processes have disposed of these impurities in landfills after they have been treated with lime or other suitable alkali substances to generate disposable precipitates. However, with increasing environmental regulation and decreasing availability of landfills, there has been a movement to find uses for the impurities, as well as to develop methods under which to render them useful.
One of the impurities that is found in sources of titanium is iron in the form of iron oxides. Many raw materials that contain titanium and a significant amount of iron oxides also contain relatively minor amounts of the other aforementioned impurities. For example, Norwegian rock ilmenite, (which is exclusively used in the sulfate process,) contains approximately 44 wt. % titanium dioxide, approximately 45 wt. % iron oxide and also approximately 0.3 wt. % MnO, approximately 0.08 wt. % Cr2O3, and approximately 0.2 wt. % V2O5. When this type of ore is processed, the iron bearing stream contains lower levels of other impurity materials than would be produced from a higher grade TiO2 ore. Thus, it is not surprising that a body of knowledge exists on the manufacture of iron oxide particles from the by-product or waste streams from such a process.
A typical beach sand ilmenite (which may be used in the sulfate process as at least part of the raw material, and may also be used as at least part of the raw material fed to a chloride process,) contains from about 55 wt. % to about 60 wt. % titanium dioxide; from about 33 wt. % to about 38 wt. % iron oxide; from about 1 wt. % to about 1.5 wt. % MnO; from about 0.04 wt. % to about 0.15 wt. % Cr2O3; and about 0.15 wt. % V2O5. Although the iron bearing stream from such a process is a little richer in the other impurity materials than in the case above, ways and means also exist for the preparation of iron oxide particles from these sources of titanium.
One known method for obtaining iron oxide particles from iron chloride solutions generated during the chloride process for the production of titanium dioxide uses excess chlorine in the chlorination step in order to generate a substantial percentage of iron (III) chloride. This chlorinated material is then subjected to successive condensing and separating process units that are operated at various temperatures. The somewhat purified iron (III) chloride that is recovered may then be reduced to iron (II) chloride, which can subsequently be treated to generate iron oxide particles. Because of the time and resources needed to accomplish these steps, this method may be undesirably cumbersome.
The recovery of iron oxide pigments from relatively pure iron chloride solutions, such as iron chloride solutions that are generated when certain titanium bearing ores that contain iron are subjected to hydrochloric acid leaching processes is also known in the art. These iron chloride solutions contain iron in the form of iron (II) chloride and may also be generated as spent hydrochloric acid liquor resulting from the manufacture of synthetic rutile from ilmenite or possibly of upgraded slag from titanium dioxide slag. However, as described below, the known methods for recovery of iron oxide pigments from these solutions all suffer from certain limitations, particularly when applied to less pure iron chloride solutions.
Under one known process for recovering iron (II) chloride from certain waste streams, various pH adjustments are first performed to remove metal chlorides other than iron (II) chlorides by the addition of a calcium containing alkali. Calcium-based alkali is used because it is cheap, and there is a readily available supply of it. According to this process, the iron (II) chlorides are then recovered and subsequently oxidized. These relatively simple methods are possible when one begins with an iron (II) chloride solution that is fairly pure. However, with an iron-containing stream that also contains a significant level of other impurities, a large amount of calcium must be introduced, which hampers further downstream processing. Consequently, these methods are not effective with poor quality chloride streams (in terms of iron content) such as the effluent stream from a standard chloride titanium dioxide process that uses natural or synthetic rutile titanium dioxide slag, and/or upgraded titanium dioxide slag or other beneficiated ores that contain titanium dioxide, such as upgraded anatase ore, where the titanium dioxide content is greater than 80 wt. % as fed into the chlorinator.
Other sources of iron-containing waste streams may arise from chloride-grade titanium slags, which typically contain approximately 86 wt. % TiO2 and approximately 10 wt. % FeO. Titanium slags may also contain approximately 1.6 wt. % MnO, approximately 0.1 wt. % Cr2O3 and approximately 0.4 wt. % V2O5. The impurity content of the iron-containing stream from such a material is considerably greater than those discussed previously. Because of the significant concentration of iron, it is particularly desirable to develop commercial ways to treat these waste streams. However, because of the relatively high percentage of other impurities relative to the iron, it is important to develop means to separate and to recover the iron compounds effectively. The present invention is particularly useful in connection with iron-containing waste streams derived from this type of source.
In addition to removing iron compounds from waste streams, it is also desirable to be able to control the form the of the iron products that are retrieved. For example, the production of iron oxide pigments such as the yellow iron oxide known as Goethite or alpha-FeO(OH) from iron sulfate solutions such as those generated via the sulfuric acid process is one well known option for reducing waste that is generated during the production of TiO2 and for generating iron oxide pigments. In that process, titanium containing ore such as Norwegian ilmenite, is subjected to the sulfate process, which will generate spent sulfuric acid liquor that contains iron sulfate in solution. This iron sulfate may be treated and recovered to generate alpha-FeO(OH). However, that process is not useful for ilmenite ores that contain higher levels of certain impurities such as manganese, chromium, and vanadium.
Thus, there remains a need to develop a cost-effective process for treating chlorination streams that contain relativity low amounts of iron relative to other impurities, and to retrieve useable iron containing products from these streams. The present invention provides a solution to this problem.
The present invention provides methods for treating iron-containing waste streams and generating metal-containing compounds such as iron oxide and manganese oxide from such raw materials. According to the present invention, waste streams from the processing of titanium raw materials by the chloride process are subjected to successive neutralization and precipitation steps in order to recover the iron compounds selectively, without accompanying these with significant levels of alkali or alkaline earth metal compounds. These iron compounds are then subjected to the oxidation, neutralization and precipitation steps that convert the iron chloride into iron-containing compounds, such as iron oxide. Following the production of the iron-containing compounds, one may also generate manganese-containing compounds such as manganese oxide.
Under one embodiment, the present invention provides a process for treating an iron-containing waste stream that is a liquid slurry stream and producing an iron-containing compound. The process comprises the steps of:
a. adding an initial neutralization agent to a liquid slurry stream to form a first precipitate and a first liquid phase, wherein said liquid slurry stream comprises an iron (II) chloride;
b. combining said first liquid phase and a sulfate-containing compound to form a second precipitate and a second liquid phase, wherein said second precipitate comprises gypsum and said second liquid phase comprises iron (II) chloride;
c. separating said second precipitate from said second liquid phase; and
d. subjecting said second liquid phase to an oxidation, neutralization and precipitation process to form an iron-containing compound and a residual liquid phase.
According to this embodiment, known as the xe2x80x9cseries process,xe2x80x9d the first precipitate may be separated from the first liquid phase prior to combining the first liquid phase and sulfate-containing compound, or the first precipitate may be separated at a later time, such as when the second precipitate is separated. In the series process, preferably the initial neutralization agent is a calcium-containing neutralization agent.
Under a second embodiment, another process for treating an iron-containing waste stream is provided. This process comprises the steps of:
a. dividing a liquid slurry stream into a first slurry stream and a second slurry stream, wherein said liquid slurry stream comprises iron (II) chloride;
b. adding a calcium-containing neutralization agent to said first slurry stream to form a metal hydroxide-containing precipitate and a calcium chloride-containing liquid phase;
c. separating a majority of said calcium chloride-containing liquid phase from the metal hydroxide-containing precipitate and a minority of said calcium chloride-containing liquid phase;
d. adding said metal hydroxide-containing precipitate and said minority of said calcium chloride-containing liquid phase to said second slurry stream to form a first precipitate and a first liquid phase;
e. separating said first precipitate from said first liquid phase; and
f. subjecting said first liquid phase to an oxidation, neutralization and precipitation process to form an iron-containing compound and a residual liquid phase.
According to this embodiment, known as the xe2x80x9cparallel process,xe2x80x9d a sulfate-containing compound may optionally be added at various points in the process in order to facilitate processing by aiding in the removal of calcium. If the sulfate-containing compound is added to the first liquid phase, the first precipitate may optionally be separated from the first liquid phase prior to combining the first liquid phase with the sulfate-containing compound. Alternatively, the first precipitate does not need to be separated from the first liquid phase prior to the addition of the sulfate-containing compound, and both precipitates may be separated during step (e). Under the latter method, the first precipitate may be separated from a second liquid phase either individually or at the same time that a second precipitate is separated from the second liquid phase.
Under a third embodiment, a process is provided for treating an iron-containing waste stream from a liquid slurry stream that comprises the steps of:
a. adding an initial neutralization agent to a liquid slurry stream to form a first precipitate and a first liquid phase, wherein said liquid slurry stream comprises an iron (II) chloride;
b. dividing said first liquid phase into a first solution and a second solution;
c. adding a calcium-containing neutralization agent to said first solution to form a metal hydroxide-containing precipitate and a calcium chloride-containing liquid phase;
d. using said metal hydroxide-containing precipitate and a minority of said calcium-chloride containing liquid phase as said initial neutralization agent; and
e. subjecting said second solution to an oxidation, neutralization and precipitate process to form an iron-containing compound and a residual liquid phase.
As with the second embodiment, in this embodiment, which is known as the xe2x80x9crecycle process,xe2x80x9d a sulfate-containing compound may optionally be added at various points in the process.
All of the aforementioned processes offer the benefit of the ability to increase the recovery of useful iron products and to generate higher purity waste liquids.